(A) Field of the Invention
This invention relates to forming a boss, i.e., an elevated area upon a surface and more particularly relates to the formation of such a boss upon an essentially planar non-porous thermoplastic polymer surface. The invention further relates to articles having such an elevated portion or boss.
(B) History of the Prior Art
The formation of protruding or elevated portions from surfaces has been of historical significance both for utilitarian and decorative purposes. Protruding portions from surfaces have been formed since before recorded history and, in fact, historical records themselves are frequently in the form of hieroglyphics or pictographs formed in elevation or relief in stone surfaces. A formation of such protruding surfaces in stone was a tedious and time consuming task, taking days, weeks or months for the completion of a single tablet. Since the advent of higher technologies, protruding surfaces were frequently formed by molding a plastic material, by etching and by metallic embossing wherein an embossing die is forced into one surface of a relatively thin sheet or plate causing an elevated portion or boss on the reverse surface of the sheet or plate.
Since thermoplastic organic polymers have been available, various methods have been tried for forming bosses using such polymers. Such methods have not been entirely satisfactory particularly when a very high gloss surface is desired. For example, such thermoplastic polymers have been molded by casting. Casting techniques, in the absence of pressure, have not always been satisfactory since the thermoplastic polymer did not always reach all mold crevices. Furthermore, mold marks caused by machining and polishing, as well as the presence of dirt on the mold surface, often unacceptably reduced the gloss of the finished article. Furthermore, the manufacture of molds for such thermoplastic polymers was exceedingly time consuming, intricate and costly. Another molding method, injection molding, wherein high pressure is used, practically eliminated the problem of the thermoplastic reaching all areas of the mold; however, the molds required for injection molding are even more costly and difficult to manufacture. Even with high polish, such molds still often do not permit a gloss as high as desirable.
Such molding methods, as above described, are particularly undesirable for the manufacture of organic plastic articles wherein small numbers of articles are desired due to the difficulty of mold manufacture and resulting high cost per article. Furthermore, when molds are used for manufacturing articles having bosses, rapid set up time to manufacture the articles is virtually impossible due to the lengthy mold manufacturing process.
Attempts have been made to manufacture organic plastic articles having bosses by pouring a solidified liquid plastic onto the surface to form a protrusion which remained due to surface tension. It was subsequently discovered that if the liquid plastic either flowed to a sharp peripheral edge or it was poured into a cavity, better relief and definition could be obtained.
More recently, in order to avoid the problems with forming cavities, a retaining edge or dam was screened onto the face of a plastic sheet. The dam then acted as a retaining wall to hold the liquid plastic. Such screening processes were still more complex than desired and did not result in retaining dams which were as efficient as desired.
It was known in the prior art that plastics could be impressed to form a debossed pattern area surrounded by the ridge at the edges of the debossed zone. See e.g. U.S. Pat. No. 2,306,256. In was not, however, recognized that a continuous multi ridge dam defining a pattern area could be formed by impressing thermoplastic surface with a debossing die.